1. Field of the Invention
This invention relates to a two-way wireless data network. More particularly, the invention relates to a two-way wireless data network, including personal mobile terminals, cellularized base stations distributed throughout a service area, and one or more network control stations.
2. Discussion of the Related Art
One-way communication systems, such as one-way paging systems, have become an increasingly important communication tool for business. Pagers are now indispensable for many people in conducting their business.
FIG. 1 shows a prior art one-way communication system 100 which is used as a one-way paging system. In a typical prior art one-way paging system, a paging party, i.e., the message sender, uses a telephone, e.g. telephone 101, to call the telephone number assigned to the paged party's pager 113. The call is received by a paging service station 103 over telephone line 102 (of course, telephone 101 could be a wireless telephone such as a cellular phone). At the prompting of paging service station 103, the paging party enters the number of his or her telephone 101. In some cases the paging party can also enter a short message at this time. Paging service station 103 converts the information received from telephone 101 into digital or analog data and transfers the data over line 105, or by wireless methods, to transmission tower 107. At transmission tower 107, a data signal 109 is generated and broadcast throughout the service area. The information that is typically included in data signal 109 is a unique identification code designating the receiving pager, and a very short message. The total size of the transmission packet is necessarily small, due to the low data rate (e.g. 2400 baud) and also because transmission tower 107 is a scarce resource shared by a large number of subscribers of the paging service over a large service area (e.g. 100 square miles). Transmission tower 107 necessarily uses high-transmission power to cover such a large service area. Further, because transmission tower 107 is shared by a large number of subscribers, not only must the messages be very short, the elapsed time between the time a message being provided to the paging network control center to the time it is transmitted can be a long delay.
When signal 109 is received by antenna 111, pager 113, which constantly monitors the broadcast channel to look for messages designated to it, alerts the paged party to the incoming signal 109 by beeping, vibrating, or some other alarm, and the telephone number of the paging party's telephone 101 (or the short message) is displayed on a display screen (not shown) of pager 113. The paged party must then respond. Responding to a paging message involves locating a second telephone 115, dialing the number displayed on pager 113, and contacting the paging party at telephone 101 over telephone line 117.
Because communication system 100 uses a single broadcast channel, and a very small number of transmitters is shared among a large number of subscribers, one-way paging network can be provided to a large number of subscribers at a user fee level which may be one-tenth the cost of cellular telephone communication. Communication protocols are also designed to allow the pager's radio receiver to "sleep" most of the time, except at pre-assigned time slots, without the risk of losing messages. Thus, a pager can typically operate on a small battery for a relatively long period of time (e.g. AAA battery operates a typical pager for a month; by comparison, a much larger battery provides several hours of use to a cellular telephone).
One-way communication networks such as the prior art one-way communication system 100 described above, are also used throughout the world for purposes other than paging. For instance, in some parts of the world, information dissemination systems, based on networks similar to one-way communication system 100, provide information such as prices of goods, stock prices, weather reports and news. Under this regime, the market, stock, weather, or news information is typically sent to service stations, similar to pager service station 103 in FIG. 1, where the information is encoded as analog or digital data. The data is then sent to broadcast station 107 where signal 109, which carries the data, is generated and broadcast. Signal 109 is then received by customers/subscribers on wireless terminals 113.
The information dissemination system based on prior art one-way communication system 100 described above is particularly useful for sending real-time information to rural or under-developed regions which are not adequately served by telephone lines and are too remote to receive standard radio or television broadcasts. Unfortunately, prior art one-way communication systems lack the ability to provide an immediate response or reply. For instance, as seen in one-way communication system 100, the paging party has no way of knowing whether the message, which is typically just the sender's phone number, was successfully transmitted or received by the paged party unless and until the paged party contacts the paging party. Further, the paged party typically cannot respond to the paging signal unless access to a hardwired or cellular telephone is available.
In addition, in a one-way communication system, the pager's lack of transmission capability severely limits its usefulness for information dissemination. For instance, because prior art one-way communication system 100 does not have an "information on demand" capability, information of potential interest to a subscriber in a broadcast region is transmitted indiscriminately to all subscribers in that region. The information is then typically repeated periodically or at regular intervals. Unfortunately, this scheme requires that each subscriber receive the same information. Typically, such indiscriminate broadcast of information results in more information than the subscriber actually wants and is inefficient. Further, because an individual subscriber does not have control over when the information is broadcast, for information having value related to its timeliness, e.g. a stock quote, the lack of "information on demand" is a severe drawback. Because of these limitations discussed above, and other limitations, one-way communication network 100 is thus limited to paging and information dissemination applications of the types discussed above.
To overcome the deficiencies of one-way paging services, two-way paging systems, are currently being developed. For example, the Motorola REFLEX system is designed to provide a reverse channel in the paging network so that pagers can acknowledge receipt of a message. FIG. 2 shows one such two-way paging system 200. Two-way paging system 200 includes: pagers 213 and 230, with antennas 211 and 231, respectively; transmission tower 207; and receiving stations 215-219.
Pager 213 includes a transmitter (not shown) so that when pager 213 receives signal 209 from transmission tower 207 (which, in this example, was initiated by a telephone call to a paging station, and leaving pager 230's identification code as the reply destination), the subscriber operating pager 213 respond to the message by transmitting a signal 220 to one of the receiving stations 215-219. It is worth noting that receiving stations 215-219 are listen-only stations and are not capable of transmitting messages back to a pager.
When one or more of receiving stations 215-219 receives signal 220 from pager 213, the receiving station or stations relay the data encoded in signal 220 to transmission tower 207 by line 222. Transmission tower 207 then broadcasts a signal 223 to the party operating pager 230.
By adding receiving stations 215-219, two-way paging system 200 solves one of the problems associated with one-way communication networks, i.e., subscriber operating pagers 213 or 230 can acknowledge receipt of a message immediately, without resorting to another communication medium such as the telephone. However, as pointed out above, data link 220 between pager 213 and receiving unit 215 is a one-way data link. Therefore, receiving station 215 cannot send any information back to pager 213. Consequently, a subscriber operating pager 213 still cannot determine whether his acknowledgement or reply signal 220 has been successfully received and forwarded by receiving station 215, until an acknowledgement message is sent via transmission tower 207. Because transmission tower 207 handles all broadcast traffic to the pagers, the acknowledgement message may be delayed for a long time. Consequently, two-way paging system 200 cannot be used for applications that require reliable data delivery confirmation. Further, because transmission of a data message depends on transmission tower 207, the bandwidth of transmission tower 207 is still limited in the same manner one-way communication network 100 discussed above. Thus, other than providing an acknowledgement capability to the pager, two-way paging system 200 suffers substantially all the disadvantages of one-way communication network 100.
In addition to the technical shortcomings of two-way paging system 200, two-way paging system 200 is also not economical. One major drawback of two-way paging system 200 is it's failure to make use of the existing one-way paging infrastructure. Instead, two-way paging system 200 operates in parallel with, and in competition with, existing one-way communication systems because two-way paging system 200 uses a different operating frequency from conventional one-way paging services which, in turn, requires separate transmission towers, signal protocols, data rates, and data formats which are incompatible with those used with one-way communication system 100. Consequently, in order for pagers 213 and 230 to communicate under two-way paging system 200, all subscribers operating wireless terminals must subscribe to the same paging service and nothing is offered to allow subscribers of two-way paging system 200 to communicate with other two-way or one-way paging services.
Since two-way paging system 200 does not utilize the existing one-way communication infrastructure, two-way paging systems, such as two-way paging system 200, have been slow to gain acceptance in the marketplace due to the large number of one-way paging systems already in use throughout the world which provide economical service and represent huge capital investments. The marginal additional capability of two-way paging system 200 does not justify the sizable additional investment necessary for such an entirely new and incompatible system. Besides, even if such an investment is made, building a new subscriber base for this system is likely to be difficult, since a minimum number or "critical mass" of customers is required before the network becomes viable. The new subscriber in prior art two-way paging system 200 faces not only the costs of both purchasing a new pager and accepting a new pager number, but also the severe constraint that two-way communications are possible only with other subscribers of the same paging service.
Two-way communication services which provides two-way communication between a cellularized receiving or base station and a mobile unit exist. For example, in a cellular digital packet data (CDPD) system, a large number of cellularized base stations are distributed all over the service area. Cellularizing the service area offers two advantages: (i) allow mobile units and base stations to transmit at higher data rate with relatively low power, since the expected distance between a mobile unit and a base station in the vicinity is short; and (ii) larger capacity is provided because base stations which are separated by large enough distances can use the same radio channels. Such a system provides thus very high capacity, low response delay and allows the mobile units to transmit at relatively high data rates. In such a system, two-way symmetrical and reliable data links can also be provided. However, such a system is expensive and complex.
In a CDPD system, because connectivity is maintained over the entire duration of a data communication session, multiple channels must be provided to allow multiple sessions to be maintained simultaneously. To locate a recipient mobile unit of a message, the network broadcasts the address of the recipient mobile unit from all the base stations in the service area until the recipient unit responds. Thus, a large amount of network resources is dedicated to locating mobile units. Further, to maintain continuous connectivity and to allow real time performance, when the session is established, the CDPD unit is associated with a base station with which it communicates. In addition, because a mobile unit can be expected to be used in a moving vehicle, it is possible that the mobile unit moves out of the service range of the initially associated base station and moves into the service range or ranges of one or more such base stations during the duration of a session. Thus, provisions must be made to disengage an associated base station and to engage an additional base station or stations ("hand off") during the course of the session. The control mechanisms for maintaining a CDPD session, including tasks typically termed "connectivity management" and "mobility management", involve sophisticated algorithms which require high performance computers to handle setting up the session, maintaining the session, and tracking the communicating mobile units as they move between service areas of the cellularized base stations. The complexity of the system requires a large investment in expensive equipment. Often, these control mechanisms are centralized, i.e. a large network switching or control center is provided to handle the mobile units in a given service area, so that, at times of heavy data traffic, the network control center may become a bottle neck, introducing undesirable latency into the system.
Another major disadvantage of the CDPD system is the requirement that the receiver of the mobile unit must be on at all times to receive messages. As a result, such a mobile unit requires a battery that is, at the present time, too undesirably bulky for mobile use. It would also be extremely difficult for power-saving features to be provided in such a mobile unit. Unlike a pager, which monitors only one paging channel, the mobile unit of a CDPD system must monitor a different radio channel when it is located in a different cell. Further, control information (e.g. timing information and recipient addresses) are broadcast by base stations in dedicated control channels. While a pager can be assigned a periodically occurring time-slot during which it "wakes" up to check for messages, a similar scheme in a mobile unit of a CDPD system would require timing in all the cellular base stations to be synchronized. Failure to synchronize all base station can lead to a mobile unit checking for the broadcast of its address during the wrong time slot.
Thus, what is needed is a method for establishing a two-way data network which (i) is high system capacity; (ii) has a low equipment cost resulting from a simple design, (iii) has a communication protocol which allows low-power operation by the wireless terminals; (iv) utilizes the existing one-way communication network infrastructure, (v) is compatible with existing one-way pagers; (vi) provides secure and reliable data transfer with short response time; and (vii) which allows bidirectional communication between mobile units without requiring sophisticated equipment to perform location, connectivity and mobility management tasks.